The devices of the present invention are of a general type including a cage or body containing a passage through which a cord or cords can extend, and a slide received at least partially within the passage and which is movable relative to the cage between a locking position preventing movement of the cord or cords longitudinally in a predetermined direction relative to the device, and a released position permitting such movement. The passage in the cage has two opposite side walls defining a locking restriction which coacts with the slide to retain the cords against longitudinal movement when the slide is in its locking position. Usually, there are two cords (or two portions of the same cord) received at opposite sides of the slide, to be clamped tightly between the slide and the side walls of the passage in the locking position of the slide.
In some devices of this general type, means have been provided for automatically moving the slide to its locking position whenever a pulling force is exerted on the cords in a predetermined direction, and without the necessity for manually moving the slide to that locking position. For example, in U.S. Pat. No. 3,965,544, the slide has two gripping arms which are yieldingly urged away from one another and against two cords to always engage those cords and be moved by the cords toward locking position when the cords move in that direction. U.S. Pat. No. 4,156,574 shows an arrangement in which the slide has a main cord gripping portion and in addition has two spring fingers for engaging the cord to follow movements thereof. In another cord locking device which is on the market, the slide is a portion of a molded plastic part having a spring portion and an integral end element which is locked in fixed position within the cage of the device in a relation causing the slide to be yieldingly urged toward its active locking position, and thus attain a self locking action.